A pneumatic tire includes a ground contacting portion or tread, the tread having a pattern designed to provide the tire with a desirable combination of traction, durability, ride comfort and quiet operation. It is also desirable that the tread pattern provide the tire with an all-weather capability, that is a set of characteristics providing adequate performance under a variety of adverse road conditions including snow, ice, rain and mud.
The all season tire had been introduced by the Goodyear Tire and Rubber Company many decades ago and was defined by lateral extending grooves open to the side of the tread. These lateral extending grooves were oriented perpendicular to the direction of travel for at least 0.5 inches and a width of at least 0.06 inches from the open shoulder laterally inward and provided a huge improvement in snow traction, virtually reducing the need for snow tires except in the most extreme weather conditions. Such tires are defined in U.S. Pat. No. 4,690,189.
Tire tread patterns designed for traction on wet surfaces, snow and ice often feature a block type tread pattern. A block type tread pattern is characterized by a plurality of main grooves extending in a circumferential direction and a number of lateral grooves extending in a more or less axial direction. The areas of tread between the circumferential and lateral grooves are referred to a tread blocks. Tread blocks may also be defined by the edges of the tread and by grooves having other orientations. In comparison, rib-type tread patterns are characterized primarily by circumferential grooves separating circumferentially continuous ribs. Tread designs may also combine rib and block patterns.
The use of blocks as elements of a tread pattern tends to increase the level of noise generated by such tires as compared to rib-type tires. Also, as noted by U.S. Pat. No. 5,538,060, such blocks have a tendency towards irregular wear due primarily to their lack of stiffness in the circumferential direction of the tread.
It is known in pneumatic tires having a block tread pattern that normal operation of the tire produces uneven wear of the tread blocks called heel-and-toe wear. In heel-and-toe wear, the rate of wear at the toe or trailing edge of the blocks exceeds the rate of wear at the heel or leading edge of the blocks. In normal operation, the heel of each block strikes the pavement first followed by the toe. Similarly the heel of each block is lifted first from its contact with the pavement followed by the toe. In addition to reduced tread life, heel-and-toe wear increases the level of noise generated by the operation of the fire. Also, the cornering and braking performance of a tire with heel-and-toe wear may be degraded.
U.S. Pat. No. 5,891,276 discloses a variation of the block tread pattern designed to suppress heel-and-toe wear wherein a narrow block is provided outside each block, the narrow block having a surface formed to be a circular arc by setting both end parts of the narrow block to be lower than the adjacent tread block by 1.5 to 2.5 mm.
In U.S. Pat. No. 6,378,583 it was disclosed to provide an improvement that is generally applicable to the design of block tread patterns for pneumatic tires and particularly applicable to directional block tread patterns having the capability of balancing heel-and-toe wear. To balance the rate of heel and toe wear, the leading edge or heel of one or more blocks are provided with one or more notches, the notches having a variable width in the axial direction, the width generally decreasing from a maximum at the heel to a minimum in the direction of the toe. Said notches provide the tread blocks with a variable net to gross where the net to gross increases from the heel to the toe of the blocks.
In another refinement of an all season tire, Goodyear introduced a series of superior rain traction tires, Aquatread and Eagle Aquatread, with directional tread patterns. In U.S. Pat. No. 5,176,766 it was reported the use of aqua-channel large circumferential grooves 11 with a width 7 to 12 percent of tread width combined with a network of generally curved inclined lateral grooves 15 flowing over the tread shoulders could greatly enhance wet traction. As shown in prior art FIG. 3, the aqua-channel 11 was connected to curved lateral grooves 15 and the water was directed into the large groove 11 and into the lateral grooves 15 to be expelled through the channel 11 or through the lateral grooves 15. It was believed important that the inclination of the lateral grooves 15 did not channel water back into the center groove 12. In U.S. Pat. No. 5,503,206 and U.S. Pat. No. 5,957,179 it was particularly noted that these directional treads should never have the lateral grooves oriented such that water is directed to the center of the tread and therefore the orientation is such the axially inner portions of a lateral groove and the leading edges 17 and trailing edges 19 of the tread elements 18 must always enter the footprint or contact patch prior to the axially outer portions accordingly any inclination other than 90 degrees had to be inclined or sloped away from the contact patch as the grooves 15 extended axially outwardly.
These design constraints while believed to improve traction, have been found to contribute to irregular heel toe wear in the shoulder block elements. This irregular wear is exaggerated or reduced depending on the shape of the tire's footprint or contact patch shape.
In U.S. Pat. No. 6,443,199 footprint shapes were determined to greatly influence tread wear. The goal in that patent was to develop tires wherein the footprint regardless of load operated in a range of footprint shape factors that would permit tire treads to be optimized to avoid the need for tire rotation to minimize tread wear. In that prior art patent which the present invention incorporates herein by reference in its entirety, measuring a tire's footprint and calculating a footprint shape factor (FSF) are explained.
After the tire is broken in using the ASTM break-in procedure for the tire the footprint shape can be determined.
To measure the footprint shape a tire is either inked and pressed against a paper or cardboard sheet which is laid on a flat hard surface at a fixed load and with the tire inflated at a fixed pressure leaving the impression of the tread on the paper or cardboard surface. This technique of footprinting is old in the tire art and is commonly understood. Alternatively, inkless procedures are also available which include carbonless paper, pressure sensing pads and the like. In all cases, one of the objectives is to get the tread contacting surfaces within the footprint defined.
Once the tire engineer has the footprint shape he or she can make several observations or predictions about the tire and its tread.
Historically, the butterfly shaped footprint was determined to be undesirable. Alternatively, the footprints having a shape similar to the bow of a boat were considered desirable for pushing water away from the center of the tread. As shown in FIGS. 1 and 2 the prior art tire exhibits this bow shape of footprint.
Inherently, when the leading and trailing edges of the footprint are not axially extending, that is if they are curved or bowed, this means that as the tire rolls a portion of the tread contacts the ground first and laterally adjacent tread elements follow. This can cause a phenomenon known as tread element squirm. As the tread elements leave the treads footprint the elements snap out of the contact patch as the pressure holding the element against the road is released. The elements lightly contacting the road are slid across the roadway wearing the element similar to sliding rubber eraser across a sheet of paper. Those inventors believed ideally the tread elements should have a uniform pressure distribution laterally across the tread and more preferably the leading and trailing edges of the footprint should be axially extending in a straight line path under all operating conditions.
To better understand this ideal relationship, they developed a concept and methodology to define the footprint shape factor which is shown in prior art FIGS. 1 and 2.
First, the maximum axial width W of the footprint is measured. Then, the distance halfway between the maximum axial width W is defined as the tire's centerplane CP. A distance 40% of the tread width (W) on each side of the centerplane is located as shown as reference numerals 2, 4. A circumferential line 5, 6 is drawn through points 2-2 and 4-4 respectively and the length of line Ls1 and Ls2 is calculated, summed and divided by 2 to arrive at an average shoulder length A. The footprint length Lc at the centerplane is measured. The footprint shape factor F is the ratio of Lc/Ls.
As shown the footprint shape factor F of the prior art tire was 1.12 at normal inflation and normal load, at the same pressure and at 50% load the footprint shape factor F is 1.50. As can be easily appreciated the footprint's shape is very different at these different loads.
In light truck tires this variation in loading is a greater problem than in passenger tires.
The present invention has remarkably found a great improvement in irregular tread wear can be achieved using lateral groove orientations that completely go against the conventional thinking of those skilled in the art of tire tread engineering and design. Furthermore, they have conducted studies confirming the use of this new inventive tread pattern design while reducing heel toe wear dramatically in the shoulder tread elements, causing at most only minor degradation in wet or dry traction performance.